1. Field of the Invention
The invention relates to an air intake control system for an engine equipped with an exhaust gas recirculation system which controls the amount of exhaust gas that is recirculated on the basis of a comparison of an air charging ratio with a target charging ratio.
2. Description of the Related Art
In order to lower nitrogen oxide (NOx) emissions in the exhaust gas from an engine, it is typical to equip an exhaust gas recirculation system which has exhaust gas recirculation adjusting means and control means for determining a target amount of fresh air charge according to an operated position of an accelerator and an air charging ratio dictated by pressure in an intake air passage and a temperature of intake air and controlling the exhaust gas recirculation adjusting means to remove a difference between a practical amount of fresh air charge detected by an air flow sensor and a target amount of fresh air charge. Such a control system is known from, for example, Japanese Patent Publication No.63-50544. This control system can be applied to gasoline engines as well as to diesel engines. In a fuel direct injection type of gasoline engine adapted to make fuel injection in a compression stroke to form uneven distribution of an airfuel mixture around a spark plug so as thereby to cause stratified charge combustion, it is potentially performed to recirculate a large amount of exhaust gas into the engine while stratified charge combustion occurs. In this case, the prior art control system to adjust amounts of fresh air charge and exhaust gas recirculation is effective.
However, the prior art control system is accompanied by aggravation of controllability of the amount of exhaust gas admitted to the engine in the event of, for example, an occurrence of a change in intake air density due to changes in atmospheric pressure and/or temperature. A reduction in intake air density caused due to a change in atmospheric pressure and temperature is accompanied by a reduction in mass of intake air if an engine operating condition and/or recirculation of exhaust gas remain unchanged. In the case where the control of exhaust gas recirculation is performed on the basis of a comparison between a practical amount of fresh air charge and a target amount of fresh air charge, if a correction is made by tempering an amount of fresh air charge with intake air density, the exhaust gas recirculation adjusting means can be controlled to make up a downward tendency of intake air due to a decline in intake air density, which is however accompanied by a reduction in exhaust gas that is recirculated. On the other hand, a rise in intake air density is accompanied by an increase in exhaust gas that is recirculated. In view of the above, therefore, there is aggravation of the control of NOx emissions and the stability of combustion.